(1 of )In this photo provided by the Sonoma Raceway, a wildfire burns behind the Sonoma Raceway on Monday, Oct. 9, 2017, in Sonoma, Calif. Wildfires whipped by powerful winds swept through Northern California early Monday, sending residents on a headlong flight to safety through smoke and flames as homes burned.
(Steve Page/Sonoma Raceway via AP)

(2 of )Flames from a wildfire consume a a three car garage at a home Monday, Oct. 9, 2017, east of Napa, Calif. Wildfires whipped by powerful winds swept through Northern California early Monday, sending residents on a headlong flight to safety through smoke and flames as homes burned. (AP Photo/Rich Pedroncelli)

(3 of )A woman takes a photo of the sun covered from smoke and haze from fires in Santa Rosa, Calif., Monday, Oct. 9, 2017. Wildfires whipped by powerful winds swept through Northern California early Monday, sending residents on a headlong flight to safety through smoke and flames as homes burned. (AP Photo/Jeff Chiu)

(4 of )A thick layer of smoke settles over West Road in Redwood Valley, California on Monday, October 9, 2017. According to reports, some residents who were unable to evacuate in time along West and Tomki Roads succumbed to the fast-moving fire. (Alvin Jornada / The Press Democrat)

(5 of )A map of hot spots observed by a NASA satellite shows the general outlines of fires in Sonoma County on Monday, Oct. 9, 2017. (Source: Arcgis.com)

(51 of ) The aftermath of the fires that have spread through Sonoma, Napa and Mendocino counties have claimed 10 lives and over 1,500 structures Photos Will Bucquoy/ for the Press Democrat).

(52 of ) Fire Aftermath. A sign in a neighborhood convenience store in Santa Rosa October 9th, 2017.The aftermath of the fires that have spread through Sonoma, Napa and Mendocino counties have claimed 10 lives and over 1,500 structures Photos Will Bucquoy/ for the Press Democrat).

(53 of ) The aftermath of the fires that have spread through Sonoma, Napa and Mendocino counties have claimed 10 lives and over 1,500 structures Photos Will Bucquoy/ for the Press Democrat).

Heavy smoke from wildfires in Sonoma and Napa counties prompted a smoke advisory throughout the Bay Area, with air quality readings in some areas well above the level considered “very unhealthy,” the region’s air board said.

The Bay Area Air Quality Management District said a monitor in Napa recorded an air quality index of 442 Monday morning, with a high of 292 in San Rafael. Air quality indexes of 201 to 300 are considered “very unhealthy,” said Kristine Roselius, a district spokeswoman.

Air quality above 100 is considered “unhealthy for sensitive groups,” and from 151 to 200 is unhealthy.

Roselius said she could not recall another time in which air quality indexes were so high for such a sustained period.

Particulate matter in smoke can irritate the eyes and airways, which can cause coughing, a scratchy throat and irritated sinuses, the district said. Smoke causes a greater risk to the elderly, children and people with respiratory illnesses.

“If it smells smoky or looks smoky, it’s a good time to stay indoors and especially to avoid exercise,” Roselius said.

Setting air conditioning to recirculate air rather than pull in outside air also is recommended, she said.

Breezy conditions, with north winds gusting more than 30 mph, continued through Monday morning, but were expected to ease overnight and shift to a flow from the south, National Weather Service meteorologist Roger Gass said.

Temperatures in the Sonoma and Napa county fire area hit the mid- to upper-80s Monday and were expected to cool by about 10 degrees Tuesday, he said.

Humidity was in the teens Monday but was expected to rise to 60 to 85 percent overnight, especially in North Bay valleys, while the hills remain drier.

Smoke will linger in the air in the two counties, especially to the west of the fires, Gass said.